If you have voxel-based statistical maps, you may be best served using a voxel-based volume renderer, like MRIcroGL
  https://www.nitrc.org/plugins/mwiki/index.php/mricrogl:MainPage
Surfice works with surface meshes, and interpolates intensity for each vertex. You could think of a vertex as an infinitely small point in space, while voxels are large discrete cubes. One therefore needs to work out some way to project between these. Surfice works best for vertex-based statistics, like those generated by FreeSurfer. For voxel based data the Surface mesh is a infinitely thin membrane, and therefore the voxels need to be projected. 
 I think the straightforward answer to your question is to uncheck "Smooth voxel-based images" from preferences, which will yield chunkier-looking surface colors with less interpolation. You may find this suitable for your data, but it may also be that other methods are better suited to your task.